Glass, multifocal lenses are conventionally produced by forming a countersink in a glass major element, fusing a segment element in the countersink, and finishing the fused blank to a desired surface curvature. It is not practical to produce an organic polymer, multifocal lens in this manner. Rather, such a lens is moulded in a one-piece construction from a single material.
Currently, organic lenses are produced by filling a cavity with a liquid monomer containing known polymerizing additives. The cavity may be formed by an assembly of two glass mould members held in spaced relationship by a flexible gasket. The inside surface contours of the two mould members are precisely the reverse of the surface contours desired on the organic lens, after shrinking of the polymer during the curing cycle.
In a mould for an organic, multifocal lens, one mould member may have a smooth, continuous, inside surface, as in a monofocal lens mould. The other mould member, here referred to as the primary mould member, must have its inside surface contoured to provide the required corrective curvatures. In a bifocal lens, there are two contours which provide the near and far vision corrections. Necessarily the corrective curvatures have different radii of curvature. Hence, they are separated at their juncture by a narrow wall extending between an edge on one surface of corrective curvature and the second surface of corrective curvature. As a result, the prescription power and surface finish quality of an organic, multifocal lens depends entirely on the finished accuracy of the glass mould members against which the lens is formed.
The conventional method of producing the primary member for an organic, multifocal lens mould is essentially a reversal of conventional, glass, multifocal lens practice. Thus, a major element is formed from a type of glass commonly known as a crown glass. This major element has a cavity, commonly referred to as a countersink, ground and polished in its concave or inside surface. The cavity is normally concave with differing curvatures as the corrective add focal length requires. The major element provides the convex curve for the far vision, or distance, correction in the final cast organic lens.
A second element, known as a segment, is formed separately for mounting in the concave cavity in the major element. In the cast organic lens, the curvature imparted by the segment portion of the mould provides near vision, or reading, correction. The segment may be of the same glass as the major element, or, more commonly, is a soft crown glass of compatible fusing properties, except 15.degree. C. to 25.degree. C. softer than the major element.
One surface of the segment is ground and polished to a curvature that is the reverse, usually convex, of the countersink curvature. The segment is then mounted in the countersink with its finished surface facing the polished countersink surface of the major. Normally, it is mounted in a predetermined location above the center of the countersink. In assembling the segment in the cavity of the major element, the lower edge rests against the cavity and the upper edge rests on spacers.
The assembly is then heated to fusion temperature to unite the segment with the major element in the countersink cavity. This fused combination is ground and polished to a predetermined far vision, or distance, curve correction and reading segment cavity size. This provides a mould member having the desired curve corrections for casting the front finished surface of an organic polymer, multifocal, ophthalmic lens.
My earlier patent, U.S. Pat. No. 4,906,422, is concerned with constructing the contoured surface on a primary mould member for a glass mould. It describes the foregoing procedure in greater detail, and, therefore, is incorporated, in its entirety, by reference. The patent is primarily concerned with a method wherein a glass segment is produced by pressing a glass body with a cavity. The glass web is removed to provide a perforated segment blank. The perforated segment blank is assembled in a concave countersink on a major element. The assembly is then fused and finished to provide a primary member for an organic, multifocal mould.
Another patent, my U.S. Pat. No. 4,992,086 describes a modified method of moulding the major element for a glass, multifocal lens blank. In particular, that application describes forming a major element that has a raised countersink zone on one surface. The remainder of the glass major element is thinner than the countersink zone, but of such thickness that it can be ground and polished to form a lens. A countersink is formed in the countersink zone and a segment element is fused in the countersink to form a lens blank assembly. The disclosure of this patent is also incorporated in its entirety by reference.